(1) Filed of the Invention
The present invention relates to a method for forming a barrel for a two-piece can. More particularly, the present invention relates to a method for forming a barrel for a two-piece can having a bottom, which has good appearance, high strength and excellent corrosion resistance. Especially, the present invention relates to a method for forming a barrel for a two-piece can having a bottom, as mentioned above, by using a thin-gauge, high-strength material.
(2) Description of the Related Art
A can body obtained by subjecting a method blank to drawing between a punch and a die and redrawing, and optionally further to ironing, is advantageous in that there is not any seam in a barrel or in a connecting portion between the barrel and bottom and the appearance is good. Further, the operations of seaming a lid and formation of a seam are not necessary and since the thickness of the side wall portion of the barrel is reduced, the amount of the metal blank can be reduced. Accordingly, this can body is widely used for the production of canned drinks and the like.
Since a two-piece can of this type is used for filling a content having a spontaneous pressure, such as beer or a carbonated drink, or for filling a content with nitrogen, a pressure resistance performance is required for the two-piece can, and especially in order to prevent buckling of the bottom, there is generally adopted a method in which an outer circumferential frustoconical portion, a protecting portion (bottom radius portion), an inner circumferential frustoconical portion and a dome portion for connecting these portions smoothly to one another are formed on the bottom toward the center thereof from the outer side.
With recent changes of the taste, in the field of such contents as carbonated drinks and beer, a higher volume ratio of carbon dioxide gas to the total volume tends to be more desired, and in the field of nitrogen-filled canned goods, in view of the filling easiness and the storage stability, a higher nitrogen volume ratio is desired. Accordingly, the pressure resistance required for a barrel for a two-piece can is at such a high level as at least 3 kg/cm.sup.2 (gauge), especially at least 6 kg/cm.sup.2. In contrast, in order to reduce the can manufacturing cost and reduce the weight of a vessel, use of a thin-gauge high-strength material is desired.
We previously proposed a method for forming a barrel for a two-piece can, having an excellent pressure in the bottom portion, from a thin-gauge high-strength material, which comprises the step of forming a barrel having a flat bottom by subjecting a metal sheet to drawing or draw-ironing, and the step of supporting the bottom of the barrel by a cylindrical punch and an annular die, having shapes corresponding to an outer circumferential frustoconical portion and a projecting portion of the bottom of the final barrel and draw-forming the bottom while engaging the bottom with a doming by having an outer diameter slightly smaller than the inner diameter of the cylindrical punch.
According to the above-mentioned method for draw-forming the bottom, a bottom having an excellent pressure resistance can be formed, but since a compressive stress is generated in the outer circumferential frustoconical portion of the bottom of the final barrel, as the gauge thickness of the metal sheet is reduced and the strength of the metal sheet is increased, the tendency of formation of wrinkles is still observed. Formation of wrinkles in the outer circumferential frustoconical portion of the bottom results in degradation of the appearance of the bottom, and in the case where a covering is formed in advance on the inner or outer surface of the metal blank, latent or actual peeling or damage is caused in the outer surface or inner surface covering material by formation wrinkles. Accordingly, formation of wrinkles should be avoided.